The home side were just a minute away from a showdown with rivals Manchester City after a Jonny Evans header, before David de Gea spilled Phil Bardsley's effort into the bottom corner.
However, Javier Hernandez netted 60 seconds later to force penalties, but in an unbelievable shootout that saw just three spot kicks scored from 10 attempts, Sunderland edged it 2-1 after Rafael missed with United's final penalty.
Read below to see how the action unfolded.
7.04pmHello, and welcome to Sports Mole's live coverage of the second leg of the Capital One Cup semi-final between Manchester United and Sunderland.
7.07pmWhen the draw was made for the semi-finals, the expectation was that Manchester United would be returning to Old Trafford with an advantage. However, after their 2-1 defeat to the Black Cats two weeks ago, David Moyes's side have a fight on their hands if they want to make it an all-Manchester final at Wembley at the start of March.
7.08pmRight then, how about some team news? Full details of both starting lineups and substitutes to follow.
7.10pmMANCHESTER UNITED XI: De Gea; Rafael, Smalling, Evans, Buttner; Januzaj, Carrick, Fletcher, Kagawa; Welbeck, Hernandez
7.17pmDavid Moyes has made five changes to the side that lost at the Stadium of Light. After netting against Chelsea at the weekend, Javier Hernandez has been handed a rare start, while Shinji Kagawa will make an appearance in midfield. Striker Wayne Rooney misses out once again with a groin problem.
7.21pmSunderland head coach Gus Poyet has opted to leave Emanuele Giaccherini on the bench, with Adam Johnson being retained after his impressive displays in the Premier League. The Italian could be an important player to throw on in the final 30 minutes of the match.
7.25pmThe Capital One Cup may be regarded as a minor competition by the majority of the top teams, but it could represent Manchester United's only realistic chance to win a trophy. They are in the last 16 of the Champions League, but on current form, they won't progress when faced with a major team, so overturning tonight's deficit could turn out to be crucial for their campaign. Lose here, and it's very much about finishing in the top four in the Premier League table.
7.29pmAs for Sunderland, a run of just one defeat in 10 games has generated some much-needed optimism on Wearside. Poyet has won a total of eight games in 19 matches in charge of the Black Cats, and a ninth this evening would earn them a place in the League Cup final for the first time since 1985.
7.30pmHowever, it should be noted that 25 games have passed since Sunderland last won at Old Trafford in a major competition, a record that stretches all the way back to 1968!
7.32pmThere's less than 15 minutes until kickoff in Manchester. How about a quick prediction?
7.34pmPREDICTION! If Manchester United can score an early goal, then you would expect them to net a couple more. However, if Sunderland can hold out for the opening 30 minutes, their confidence will grow, and given their current form, it wouldn't surprise me if they can nick a 1-1 draw, or at the very least, get through to extra-time with a 2-1 defeat.
7.37pmFor those of you who don't know, Manchester United are closing in on a bumper deal to sign Chelsea's Juan Mata, with an offer of £35m reportedly being offered for the Spaniard.
7.42pmThe two teams are heading onto the pitch at Old Trafford. Apparently, 9,000 Sunderland supporters are in Manchester. Great effort! Can they cheer on their team to a memorable triumph? Only time will tell.
3 minUnited have dominated possession in the early stages, but it's Sunderland who threaten first. A break down the left from Marcos Alonso sees the Spaniard drill a cross into the box, but it's clear by the home side.
5 minThe home side have a free kick after an extremely late challenge on Adnan Januzaj from Fabio Borini. He's fortunate not to earn a booking.
6 minSAVE! Vito Mannone has been linked with a move to Manchester United, but he denies his reported suitors with a stunning save from Javier Hernandez, who had rose above the Sunderland defence to get a header in on goal from six yards.
10 minKi Sung-Yueng tries his luck from 30 yards, but his effort is lacking in power and it trickles wide. The weak nature of the shot almost saw Adam Johnson nip in and deflect the ball on target.
13 minApart from that one Hernandez chance, Sunderland have started well. They don't look overawed by the occasions, and United are going to have to work hard to break the Black Cats down.
16 minSunderland are having plenty of joy down the left through Alonso, but his final ball has left a lot to be desired. However, if Manchester United keep giving him time and space, one of these opportunities will pay off.
19 minCHANCE! Fabio Borini breaks clear of the Sunderland midfield, but he is surrounded by several defenders. However, he opts to let fly with a dipping volley from 25 yards that only just clears David De Gea's crossbar.
23 minSunderland are receiving plenty of encouragement here. Lee Cattermole receives the ball in a two-on-two situation, but he is eventually forced to turn back and play the ball back towards his own goal. The home side need greater urgency in midfield, because that's where they are losing the ball.
26 minUnited attempt to offer a response, but they are being prevented in running up a head of steam by a resilient Sunderland backline. The noise from the crowd has increased, but it's more out of hope than expectation. This isn't a typical Manchester United performance, or maybe it is now?
30 minManchester United work a crossing opportunity down the left but Alexander Buttner's delivery is deflected into the hands of Vito Mannone. We have reached that 30-minute mark, which I believed was crucial to Sunderland. Let's see how they do from here.
34 minSunderland earn a free kick on the right-hand side, which Adam Johnson is showing a great deal of interest in. It's poor from the former Manchester City man, though, and it's headed clear by Shinji Kagawa.
37 minGOAL! Manchester United 1-0 Sunderland (Evans)
37 minAfter Darren Fletcher struck a post a minute earlier, United take the lead. A corner was flicked on by Danny Welbeck for Jonny Evans, who was waiting at the back post to nod in for three yards.
41 minFabio Borini does well to work a corner off the shins of Rafael. It's a fine delivery from Adam Johnson, with Wes Brown seeing a header cleared by Alexnader Buttner. It could have been going wide, but Buttner wasn't to know.
43 minDanny Welbeck has pulled up with what appears to be a thigh or hamstring injury. That could be his game finished.
45 minScratch that, Welbeck looks to have recovered. He looked in some trouble a minute or two ago. It's all Sunderland at the end of this first half, but they don't look like testing De Gea.
45+1 minHALF TIME: Manchester United 1-0 Sunderland
8.32pmIt's been a disappointing first half at Old Trafford, but Manchester United have the advantage after that Jonny Evans header. However, Sunderland have had their moments, and it would be foolish to think that this is now a foregone conclusion.
8.37pmThe half-time statistics don't really do Sunderland any justice. United have had 55% of possession as well as having seven shots on goal to Sunderland's four. The visitors are edging it on corners, but they are going to need a goal at some stage tonight if they want to retain a chance of reaching Wembley.
8.40pmSunderland have plenty of options on the bench. Jozy Altidore, Emanuele Giaccherini and Sebastian Larsson could all be introduced if they can't find the back of the net in the opening 15 minutes after the break.
8.45pmNeither manager has made any changes at half time. I'm sure a couple could follow if we have an early goal...
47 minUnited immediately win a free kick on the edge of the Sunderland penalty area. It's Adnan Januzaj who steps up, but his left-footed effort clears the bar by a couple of yards.
50 minShinji Kagawa has had another quiet game for United, but he plays an excellent ball down the right for Rafael, who wins a corner. From the resulting set piece, the Brazilian received the ball on the edge of the area before driving a shot marginally over the bar.
52 minIt's Sunderland's turn to break forward, and the bravery of Steven Fletcher earns his team a corner. A threatening cross is excellently headed out by Chris Smalling, who has to react fastest once more to deny John O'Shea a clear opportunity.
53 minEFFORT! Adam Johnson collects the ball on the edge of the penalty area, before driving the ball marginally wide. It took a slight deflection off Michael Carrick, but the nick wasn't seen by the referee. Understandably, Johnson isn't best pleased.
55 minSHOT! Shinji Kagawa is gradually becoming more influential in this game, and his trickery in the middle of the pitch starts a sweeping move. The break concludes with Adnan Januzaj firing the ball inches past the near post, but he will feel as though he should have done better.
59 minThis is turning into a cracker of a second half, and Sunderland are more than in this. A neat spell of possession ends with an over-ambitious ball into the box from a central position, but Gus Poyet will be happy with what he is witnessing.
61 minSUBSTITUTION! David Moyes flinches first and makes his first change. Shinji Kagawa has been replaced by Antonio Valencia.
65 minBOOKING! Fabio Borini goes into the book for a kick on Jonny Evans after the whistle.
69 minCHANCE! Antonio Valencia has everyone inside Old Trafford on their feet with a powerful run down the right, but after pulling the ball back for Adnan Januzaj, the attacking midfielder sliced his shot disappointingly wide of the post.
73 minCLOSE! Adnan Januzaj is proving once again that he is an immense talent, and he nearly doubles United's tally after cutting inside from the right, only to see his shot deflected over the bar. It was very similar to that of Samuel Eto'o's goal against United at the weekend.
75 min15 minutes remain at Old Trafford. At this stage, I should remind you that away goals only count AFTER extra-time, so as it stands, we will be having an extra 30 minutes.
76 minSunderland enjoy a superb period of play deep in the Manchester United half, and the home side are defending excellently to keep hold of a clean sheet. Adam Johnson and Fabio Borini are providing a great outlet for the Black Cats.
80 minEFFORT! What an effort! Marcos Alonso picks up the ball in an innocuous position on the left, but he surprises David de Gea with a stunning left-foot volley that fizzes inches wide of the far post. That was close!
82 minSUBSTITUTION! Gus Poyet finally decides to make his first change, bringing off Lee Cattermole for Craig Gardner.
84 minSUBSTITUTION! Patrice Evra is coming on for Alexander Buttner, who appears to be struggling with an injury. Just one more left remaining for Moyes.
86 minThe changes are coming thick and fast now. Fabio Borini, who suffered a knock a couple of minutes ago, is replaced by Jozy Altidore. Four minutes to go at Old Trafford!
88 minThe substitutions have disrupted Sunderland's growing dominance, and it is United who are finishing the stronger. They earn a corner on the right, but it's easily cleared by the visitors. Can they take it to extra time?
90 minThere will be three minutes of added-on time.
90+1 minSunderland work the ball down the right to earn a throw by the corner flag. Phil Bardsley whips a ball into the box, and although it is missed by the United defenders, it doesn't fall for a Sunderland attacker.
90+3 minManchester United earn a free kick 20 yards from goal after a driving run from Javier Hernandez. They couldn't, could they...?
90+4 minMannone just about claims the ball as it was heading into the top corner! We have extra-time.
90+4 minFULL TIME: Manchester United 1-0 Sunderland
9.37pmManchester United could have stolen that with that last-gasp free kick, but it would have been extremely harsh on Sunderland, who deserve a crack at extra time.
94 minUnited have made a fast start to extra-time, but Sunderland hold firm. There looks a few weary legs out there. Both sides have one change remaining, so they will need to use it wisely.
95 minManchester United have been forced to withdraw Michael Carrick from the action after the midfielder went down with an ankle problem in the technical area. Phil Jones is on in his place.
96 minCLOSE! Steven Fletcher wins a free kick for Sunderland on the edge of the Manchester United penalty area. Craig Gardner takes responsibility for the set piece, but sends the ball just over the bar.
100 minAdam Johnson and Phil Bardsley combine to earn a corner kick after a sliced clearance from Phil Jones. David de Gea comes out to punch the ball clear, but the visitors have the ball once more, with Bardsley dragging a long-range shot wide.
102 minCHANCE! United should be ahead. Javier Hernandez is played in behind the Sunderland defence and he is under no pressure. Surely he must score? He doesn't even hit the target after slicing a left-foot shot wide. He looks embarrassed after that.
104 minSunderland are making a terrific fight of this. Ki combines with Steven Fletcher, only for the Scot to see his shot deflected wide for a corner.
105 minHALF TIME: Manchester United 1-0 Sunderland
106 minWe are back underway. 15 minutes to go. How will this pan out? Your guess is as good as mine!
109 minJozy Altidore completely misses his kick when advancing into the penalty area, but he manages to win a throw for Sunderland, who maintain possession and push United back. The home side are holding on!
110 minBOOKING! United break through the impressive Adnan Januzaj, but he is cynically pulled back by Craig Gardner, who is deservedly booked.
113 minThis is end-to-end stuff! Adnan Januzaj accelerates towards goal, but instead of passing to Phil Jones, he opts to shoot from 20 yards, only to see it saved by Vito Mannone. Up the other end, Ki Sung-Yueng jinks past a couple of players and wins a corner for his side.
116 minJozy Altidore powers past a couple of defenders before earning yet another corner. Steven Fletcher heads over from the cross, and Sunderland still haven't had a shot on target!
118 minSunderland come so close to securing a place at Wembley. Phil Bardsley turns inside the box before firing the ball agonisingly across the box where no-one is there to meet it. Sunderland have two minutes to save themselves. United are still holding on!
119 minGOAL! Manchester United 1-1 Sunderland (Bardsley)
119 minUNBELIEVABLE! I don't believe what I have just witnessed. Phil Bardsley's 20-yard effort was straight at David de Gea, but the Spanish stopper has smothered the ball into his own net. But wait...another goal...
120+1 minManchester United 2-1 Sunderland (Hernandez)
120+1 minOh my word. Wow. Adnan Januzaj has saved David de Gea's blushes by working his way down the left before crossing for Javier Hernandez, who had an open goal, but proceeded to hit the ball into the net off the bar.
120+2 minFULL TIME: Manchester United 2-1 Sunderland
10.15pmPenalties coming up. I am stunned. Sunderland are stunned. Can they pick themselves up?
10.19pmManchester United 0-0 Sunderland Craig Gardner sends Sunderland's first penalty over the bar!
10.20pmManchester United 0-0 Sunderland Danny Welbeck does the same thing!
10.21pmManchester United 0-0 Sunderland Unbelievable! David de Gea saves from Steven Fletcher.
10.21pmManchester United 1-0 Sunderland Darren Fletcher gives United the lead. Big penalty coming up for Sunderland.
10.22pmManchester United 1-1 Sunderland Marcos Alonso gets Sunderland on the board with a fantastic spot-kick.
10.23pmManchester United 1-1 Sunderland Vito Mannone saves from Adnan Januzaj! It's 1-1 after three penalties each.
10.23pmManchester United 1-2 Sunderland Ki Sung-Yueng gives Sunderland the lead after sending De Gea the wrong way.
10.24pmManchester United 1-2 Sunderland Phil Jones blazes over! Unbelievable. It will be Adam Johnson to step up to put Sunderland into the final.
10.25pmManchester United 1-2 Sunderland De Gea saves! It's a poor penalty from Adam Johnson.
10.26pmManchester United 1-2 Sunderland Rafael misses! Absolute scenes at Old Trafford. Sunderland are in the final!
10.27pmSUNDERLAND REACH THE CAPITAL ONE CUP FINAL!
10.28pmAfter the most astonishing end to a match that you could imagine, we witnessed the worst set of penalties that you are ever likely to see. Sunderland won't care. They are going to Wembley!
10.29pmThree penalties were scored out of 10. THREE!
10.30pmThe Sunderland supporters are going crazy. Shirts are off and everything. That's how much it means to them.
10.31pmFor clarification, Rafael's penalty was saved by Vito Mannone, who has really blossomed into a top goalkeeper at the Stadium of Light.
10.33pmThat will be all from Sports Mole for this match, but please, please keep your eye out for all of the reaction from Old Trafford. Goodbye for now.